1. Field of the Invention
The present invention relates to a wavelength converter for generating from input light of a first wavelength, converted light of a second wavelength by making use of a nonlinear optical phenomenon.
2. Related Background Art
It is known in general that during propagation of high-power light in media various nonlinear optical phenomena occur because of nonlinear polarization in the media. Among these nonlinear optical phenomena, Four-Wave Mixing (FWM) is caused by the third-order nonlinearity effect and, specifically, is a phenomenon in which input of three photons into a medium results in making a new photon. When the conservation of energy and the conservation of momentum both hold among the photons associated with the nonlinear optical phenomenon, the nonlinear optical phenomenon occurs at the highest efficiency.
Research has actively been made heretofore to positively cause the above-stated nonlinear optical phenomenon in an optical fiber and apply the optical fiber to wavelength conversion and others. For example, a wavelength converter is an optical device that generates from input light of a first wavelength, converted light of a second wavelength carrying the same information as the input light does. Such wavelength converters are applied, for example, to an optical communication network in which multiple nodes are mutually connected by an optical fiber transmission network, and they are provided in these nodes. A wavelength converter in a node operates to output converted light resulting from wavelength conversion from the wavelength of the incoming input light, as output light.
There are highly nonlinear fibers readily inducing the above-discussed nonlinear optical phenomenon inside; for example, Document 1 (Jiro Hiroishi, et al., “Dispersion slope controlled HNL-DSF with high γ of 25 W−1 km−1 and band conversion experiment using this fiber,” ECOC 2002, PD1.5) discloses the high nonlinearity fiber with the reduced dispersion slope as small as 0.013 ps/nm2/km, and Document 2 (Toshiaki Okuno, et al., “Generation of Ultra-Broad-Band Supercontinuum by Dispersion-Flattened and Decreasing Fiber,” IEEE PHOTONICS TECH. LETT., VOL. 10, NO. 1, January 1998, pp.72–74) discloses the highly nonlinear dispersion-flattened fiber. Document 3 (K. P. Hansen, et al., “Fully Dispersion Controlled Triangular-Core Nonlinear Photonic Crystal Fiber,” OFC 2003, PD2) describes the dispersion-flattened highly nonlinear photonic crystal fiber with a short effective length because of large loss. Document 4 (Ju Han Lee, et al., “Four-Wave Mixing Based 10-Gb/s Tunable Wavelength Conversion Using a Holey Fiber With a High SBS Threshold,” IEEE PHOTONICS TECH. LETT., VOL. 15, NO. 3, March 2003, pp.440–442) discloses the wavelength converter making use of the holey fiber, in which the permissible wavelength difference between signal light and pumping light is only about 10 nm, because of a large absolute value of chromatic dispersion. Document 5 (K. Inoue, “Arrangement of fiber pieces for a wide wavelength conversion range by fiber four-wave mixing,” OPTICS LETTERS, VOL. 19, NO. 16, Aug. 15, 1994) discloses the technology of cascading a plurality of optical fibers with different zero-dispersion wavelengths and thereby expanding the bandwidth to about 2 THz, and Document 6 (M. Onishi, et al., “Highly Nonlinear Dispersion-Shifted Fibers and Their Application to Broadband Wavelength Converter,” OPTICAL FIBER TECHNOLOGY, VOL. 4, 204–214 (1998) discloses examples of highly nonlinear fibers.